Information is limited regarding the interactions between mechanical function, substrate oxidative metabolism and coronary vascular responsiveness, in neonatal hearts subjected to conditions of "critical O2 availability" (approximate complete O2 extraction). Such information is of great clinical importance, since seriously-ill infants with respiratory distress syndrome or congenital hear disease can suffer from severe O2 insufficiency and myocardial under perfusion. The consequences of critical O2 availability on neonatal myocardial performance will be investigated using isolated, isovolumically-beating piglet hearts. Hearts will undergo retrograde aortic perfusion with a non-recirculating, erythrocyte-enriched solution containing glucose, lactate and palmitate. Myocardial performance will be assessed by: Left ventricular peak systolic pressure and end diastolic pressure, coronary flow, myocardial O2 consumption, lactate, H+ and CPK production, intramyocardial pH, and oxidation of exogenously-supplied glucose, lactate and palmitate (by measuring 14CO2 production from the 14C-labeled substrate). Myocardial biochemical composition will be investigated by: Freeze-clamping hearts at selected times during experiments and analyzing tissue for ATP and CP, glycogen and lactate, triglyceride, long chain acyl-CoA and acylcarnitine, and pyruvate dehydrogenase activity, in order to correlate derangements in mechanical function and substrate utilization with metabolic intermediates. The aims of this proposal are to: 1) Define states of critical O2 availability by reducing coronary perfusion pressure or coronary flow to produce global myocardial ischemia (low coronary flow and adequate arterial [O2]) or by decreasing arterial O2 to produce myocardial hypoxia (elevated coronary flow and low arterial [O2]). Effects of ischemia will be contrasted to those of hypoxia by comparing results from experiments for which O2 delivery (coronary flow x [O2]) is equivalent at the onset of each process. 2) Establish that fatty acids are an important metabolic substrate involved in energy production and control of substrate utilization, but which may also have deleterious effects on mechanical function in O2-limited neonatal hearts. Fatty acid metabolism will be further characterized by myocardial triglyceride degradation and endogenous triglyceride fatty acid oxidation. 3) Demonstrate that glucose utilization has beneficial effects on neonatal hearts exposed to profound ischemia or hypoxia. Glucose utilization will be enhanced by increasing glycolytic flux with additional glucose and insulin or by increasing glucose oxidation with dichloroacetate, which stimulates pyruvate dehydrogenase. Glucose flux will be assessed by determining 3H2O production from 3H-glucose and glucose oxidation by 14CO2 production from 14C-glucose. 4) Establish that critical O2 availability impairs coronary flow regulation in neonatal hearts through cyclooxygenase product release and/or an interdiction of the nitric oxide pathway. The former and latter processes will be investigated by supplementing the perfusate with indomethacin/SQ29548 and N omega-nitroarginine, respectively.